The present invention relates to a device for measuring the pressure of blood.
More particularly, the present invention relates to a device for measuring the pressure of blood, which is used in an extra corporeal blood treatment device in which the blood is taken from a patient in order to be treated then reintroduced into the body of the patient (especially for the purpose of carrying out dialysis) by means of an extracorporeal blood circuit comprising pipes and including at least one section for measuring the pressure of blood flowing in a pipe.
A known type of pressure measurement section comprises, in a substantially rigid wall, a hole which is sealed by a closure element, the internal face of which is in contact with the blood and the external face of which is in contact with the ambient air, it being possible to elastically deform or displace the closure element overall along a deformation or displacement axis which is substantially orthogonal to its general plane, under the effect of the blood pressure; a portion of the external face of the closure element, in its rest state, is in direct or indirect contact with a load sensor which can measure the force applied axially to the internal face of the closure element by the pressure of the blood, in order to calculate therefrom the value of this pressure.
Generally, this type of extracorporeal blood treatment device comprises a circuit part which is made from a casing, or cassette, of the disposable type, incorporating pipes which are connected to the extracorporeal blood circuit.
The pressure measurement section may be an attached module which is mounted in an associated housing of the casing.
The casing is mounted on a support apparatus which comprises, for example, sensors, display means, pumping means, a control interface, an electronic control unit, etc.
In this type of extracorporeal blood treatment device, the blood pressure must be measured without contact between the measurement member and the blood.
Several systems for carrying out this pressure measurement are known.
In a first pressure measurement system, which is shown in FIG. 1, a pressure measurement section 10 in a pipe 12 comprises a measurement chamber 14 in which a membrane 16, or diaphragm, separates the blood flowing in the pipe 12 from the air contained in a compartment 18.
The membrane 16 can be deformed along a deformation axis Axe2x80x94A which is orthogonal to its general plane, so that it is displaced axially according to the pressure of the blood in the pipe 12.
The extreme deformation positions of the membrane 16 are shown by dotted lines.
The air compartment 18 is sealed shut when the pressure measurement section 10 is mounted on a support apparatus 20.
The support apparatus 20 comprises a sensor 22 which directly measures the pressure in the air compartment 18.
When the blood pressure changes, the membrane 16 is axially displaced to an equilibrium position in which the pressure on both sides of the membrane 16 is equal.
The pressure measured by the sensor 22 in the air compartment 18 is therefore equal to the pressure of the blood in the pipe 12.
By virtue of a suitable geometry, in particular by virtue of a suitable volume for the compartment 18 and a suitable surface-area for the membrane 16, this first pressure measurement system makes it possible to measure, on the one hand, so-called xe2x80x9cpositivexe2x80x9d blood pressures, that is to say, blood pressures which are greater than a reference pressure, in this case atmospheric pressure, and, on the other hand, so-called xe2x80x9cnegativexe2x80x9d blood pressures, that is to say blood pressures which are less than the reference pressure.
This measurement system operates correctly provided that there are no leaks in the air compartment 18, otherwise the membrane 16 is displaced right up to its end stop and it no longer carries out the function of transmitting pressure.
The seal of the air compartment 18 during mounting of the pressure measurement section 10 on the support apparatus 20, is a weak point of the measurement system.
In particular, the seal may be impaired during use of the measurement system.
In a second pressure measurement system, which is shown in FIG. 2, the pressure measurement section 10 forms a compartment 24 containing the blood and one wall 26 of which comprises a hole 28 which is sealed by a flexible membrane 30.
When the pressure measurement section 10 is mounted on the support apparatus 20, the external face of the central part of the flexible membrane 30 is in contact with a load transmitter 32 which is inserted between the membrane 30 and a load sensor 34.
The load sensor 34 makes it possible to measure the forces applied to the internal face of the membrane 30 because of the effect of the blood pressure in the compartment 24, where the blood pressure is greater than the ambient air pressure.
The blood pressure is determined by the equation:                     P        =                              F            -                          F              0                                            S            a                                              (        1        )            
In this equation, F is the force measured by the load sensor 34, F0 is the force measured in the rest state, that is to say, in the absence of a pressure gradient between the two sides (internal and external faces) of the membrane 30, and Sa is the active area or active surface area of the membrane 30.
The active surface area Sa of the membrane 30 is equivalent to an area intermediate between the total area of the internal face of the membrane 30 in contact with the blood and the area of contact between the membrane 30 and the load transmitter 32.
This measurement system allows positive pressures to be measured but it does not allow negative pressures to be measured.
This is because, for negative pressures, the membrane 30 tends to come away from the load transmitter 32. The load sensor 34 can then no longer measure the forces which are applied to the membrane 30.
This system has therefore been adapted to measure negative pressure.
In order that the load sensor 34 can continue to measure the forces which are applied to the membrane 30, when the blood pressure is negative, the membrane 30 is secured in axial displacement to the load transmitter 32.
Thus, according to one improved embodiment of the second pressure measurement system, which is shown in FIG. 3, the membrane has a metal disc 36 on its external face and the load transmitter 32 has a magnet 38 at its axial end facing the membrane 30.
The magnetic attraction exerted by the magnet 38 on the metal disc 36 makes it possible to secure the membrane 30 in axial displacement to the load transmitter 32.
When the pressure is positive, the membrane 30 exerts a force which pushes axially against the load transmitter 32.
When the pressure is negative, the membrane 30 exerts a force which axially pulls the load transmitter 32.
This device for securing the membrane 30 to the load transmitter 32 is expensive since it requires a special membrane 30 fitted with a metal disc 36 and a special load transmitter 32 fitted with a magnet 38.
The metal disc 36 must have a large area in order to allow effective magnetic coupling.
In addition, the membrane 30 is subject to a significant jolt when the metal disc 36 xe2x80x9csticksxe2x80x9d to the magnet 38 of the load transmitter 32, which may impair its mechanical characteristics.
Moreover, it is noted that the known measurement systems require an attached membrane 30, which is made from a material different to that of the pressure measurement section 10.
The two pressure measurement systems generally use flexible membranes 30 made of silicone.
An attached membrane 30 is relatively complex to mount since the membranes 30 must completely seal the hole 28 of the associated wall 26, which involves high manufacturing and assembly costs for the pressure measurement system.
The purpose of the invention is to remedy these drawbacks and to provide a pressure measurement system which is simpler than the existing systems.
For this purpose, the invention proposes a device for measuring the pressure of blood in a pipe of an extracorporeal blood circuit, comprising a pressure measurement section having a substantially rigid wall including a hole which is sealed by a closure element, the internal face of which is in contact with the blood and the external face of which is in contact with the ambient air, it being possible for the closure element to be elastically deformed or displaced overall along a deformation or displacement axis which is substantially orthogonal to its general plane, under the effect of the blood pressure, the pressure measurement section being designed to engage with a load sensor so that a portion of the external face of the closure element is, in its rest state, in direct or indirect contact with a load sensor which can measure the force applied axially on the internal face of the closure element by the blood pressure, in order to calculate therefrom the value of this pressure, characterized in that the closure element is made in a single piece with the associated rigid wall of the pressure measurement section.
Other than its manufacturing cost, which is less than that of a flexible membrane attached to a pressure measurement section, the closure element according to the invention makes it possible to overcome specific problems connected with the use of flexible membranes.
To be precise, it has been observed that when a given sustained force is applied to a flexible silicone membrane, a phenomenon of creep appears, that is to say that there is a deterioration in the elastic properties of the membrane over time.
After a given period, the membrane therefore remains deformed in spite of a return to the initial conditions corresponding to its rest state.
This phenomenon of creep is particularly significant for large diameter membranes, that is to say membranes whose diameter is greater than 25 millimeters, made of silicone or of natural rubber, which lose about eight percent of their elasticity in one hour.
The temperature and the hydration of the membrane may also cause deterioration of its properties, particularly its elasticity.
According to other characteristics of the invention:
the closure element includes a region of lower resistance to elastic axial deformation, compared to the rigid wall;
the region of lower resistance to elastic axial deformation circumscribes the portion of the external face of the closure element which, in its rest state, is in direct or indirect contact with the load sensor;
the closure element comprises a substantially rigid central pellet which is delimited by a thinned peripheral annular region of axial thickness less than the axial thickness of the rigid wall in order to form an elastically deformable region;
the thinned region is made by machining the rigid wall associated with the closure element;
the closure element is made by moulding with the associated rigid wall;
in cross section on a plane which is substantially perpendicular to the general plane of the closure element, the profile of the thinned region is substantially undulating;
a load transmitter is inserted axially between the external face of the closure element and the load sensor;
the load sensor, or the load transmitter, applies an initial axial pretensioning force to the closure element, in its rest state, for the purpose of making it possible, in particular, to measure a pressure less than the pressure of the ambient air or to measure a reduction in pressure with respect to a reference pressure;
the external face of the closure element comprises a gripping member, or a member that can be gripped, which engages with a complementary member of the load transmitter, so as to secure the closure element in axial displacement with the load transmitter, for the purpose of making it possible, in particular, to measure a pressure less than the pressure of the ambient air or to measure a reduction in pressure with respect to a reference pressure;
the gripping member, or the member which can be gripped, of the closure element is made in a single piece with the pellet;
when a part of the circuit is made up of a casing, or cassette, incorporating pipes which are connected to the extracorporeal blood circuit, the pressure measurement section is an attached module which is mounted in an associated housing of the casing;
when a part of the circuit is made up of a casing, or cassette, incorporating pipes which are connected to the extracorporeal blood circuit, the pressure measurement section is moulded into the casing;
the closure element is substantially disc-shaped;
at least one portion of the rigid wall bordering the hole bears axially towards the outside against a support plate.